Cemetery categories should be based on the governing unit of that land. Based on a quick search, the CWGC is a plan (aka section, area) within Sale Public Cemetery, so it would not have its own category, and you are free to add Boys-540 to the existing category.
In regards to documentation (free-space) of the cemeteries, you have more flexibility here, and can have two (or more) pages as needed. As an example, the main FSP for the cemetery would be "Sale Cemetery, Sale, Victoria". The war plan can be documented on a separate FSP.